The word 'prion'means 'infectious protein', a protein which can transmit a disease or trait without the necessity for an accompanying nucleic acid. The prion concept has its origins in studies of the mammalian transmissible spongiform encephalopathies (TSEs), a group of uniformly fatal diseases whose underlying cause appears to be the formation of amyloid composed of the PrP protein. The mammalian prion diseases are not only uncurable, they are so far completely untreatable. Although these are rare diseases, affecting about 1 in a million people per year world wide, they are likely similar in mechanism to the many common amyloid-associated diseases such as Alzheimer's disease, Parkinson's disease, type 2 diabetes mellitus and amyotrophic lateral sclerosis. In 1994, we discovered two prions of the yeast Saccharomyces cerevisiae, called URE3 and PSI, based on a self-propagating inactivation of the Ure2 and Sup35 proteins, respectively(1). Ure2p is a regulator of nitrogen catabolism and URE3 strains show a derepression of the genes normally repressed by this protein. Sup35p is a subunit of the translation termination factor, and PSI+ strains show elevated readthrough of translation termination codons, a phenotype similar to that of a sup35 mutant. Studies by many labs have shown that the yeast prions URE3 and PSI are based on self-propagating amyloid forms of Ure2p and Sup35p, respectively (reviewed in 2). Chaperones play an important role in prion propagation, including the disaggregating chaperone Hsp104 (3), the Hsp70 family of chaperones (4) and the Hsp40s (5). Yeast prions can be cured by low concentrations of the specific Hsp104 inhibitor, guanidine (6), by overexpression of certain fragments of the prion protein (7), by inactivation of Hsp70s (4) or by overexpression of the Hsp40-group chaperone Ydj1p (8). We sought to determine if overexpression of other yeast proteins could cure the URE3 prion, screening for high-copy clones of yeast DNA that eliminate the prion (9). We found that overproduction of the chaperone, Sse1p, can efficiently cure the URE3 prion (9). Sse1p is also a nucleotide exchange factor for the Hsp70 family chaperone Ssa1p, promoting conversion of Ssa1p*GDP to Ssa1p*GTP. Fes1p has a similar activity. We found that deletion of either SSE1 or FES1 completely blocked URE3 propagation, while having little if any effect on cell growth. In addition, deletion of SSE1 also interfered with PSI+ propagation. Thus, an optimal level of Sse1p is necessary for propagation of the URE3 prion (9). Sse1p has both chaperone activity (requiring ATPase) and nucleotide exchange activity with Ssa1p (that requires ATP binding, but not ATPase). We find that a mutant that eliminates ATPase activity but retains ATP binding can still cure URE3, suggesting that it is the nucleotide exchange activity of Sse1p, not its chaperone activity, that is responsible for the curing. Since the Ssa's (cytoplasmic Hsp70s) are known to be required for yeast prion propagation, we believe that Sse1p affects prions by regulation of the Ssa's (9). Most recently, we found that overproduction of Btn2p, or its homolog Ypr158 (Cur1p), cures URE3 (10). Btn2p is reported to be associated with late endosomes and to affect sorting of several proteins. We find that double deletion of BTN2 and CUR1 stabilizes URE3 against curing by several agents, produces a remarkable increase in the proportion of strong URE3 variants arising de novo and an increase in the number of URE3 prion seeds. Thus normal levels of Btn2p and Cur1p affect prion generation and propagation. Btn2p-GFP fusion proteins appear as a single dot located close to the nucleus and the vacuole. During the curing process, those cells having both Ure2p-GFP aggregates and Btn2p-RFP dots display striking co-localization. Btn2p curing requires cell division, and our results suggest that Btn2p is part of a system, reminiscent of the mammalian aggresome, that collects aggregates preventing their efficient distribution to progeny cells (10). This information about factors determining prion stability should eventually lead to therapy approaches for human amyloid diseases. We are now characterizing other proteins whose overproduction was found to cure the URE3 prion. References: 1. Wickner, R. B. URE3 as an altered URE2 protein: evidence for a prion analog in S. cerevisiae. Science 264, 566 - 569 (1994). 2. Wickner, R.B., Edskes, H.K., Ross, E.D., Pierce, M.M., Baxa, U., Brachmann, A., and Shewmaker, F. (2004). Prion Genetics: New Rules for a New Kind of Gene. Ann. Rev. Genetics 38, 681-707. 3. Chernoff, Y.O., Lindquist, S.L., Ono, B.-I., Inge-Vechtomov, S.G., and Liebman, S.W. (1995). Role of the chaperone protein Hsp104 in propagation of the yeast prion-like factor psi+. Science 268, 880 - 884. 4. Jung, G., Jones, G., Wegrrzyn, R.D., and Masison, D.C. (2000). Arole for cytosolic Hsp70 in yeast PSI+ prion propagation and PSI+ as a cellular stress. Genetics 156, 559-570. 5. Moriyama, H., Edskes, H.K., and Wickner, R.B. (2000). URE3 prion propagation in Saccharomyces cerevisiae: requirement for chaperone Hsp104 and curing by overexpressed chaperone Ydj1p. Mol. Cell. Biol. 20, 8916 - 8922. 6. Jung, G., Jones, G., and Masison, D.C. (2002). Amino acid residue 184 of yeast Hsp104 chaperone is critical for prion-curing by guanidine, prion propagation, and thermotolerance. Proc. Natl. Acad. Sci. U. S. A. 99, 9936 - 9941. 7. Edskes, H.K., Gray, V.T., and Wickner, R.B. (1999). The URE3 prion is an aggregated form of Ure2p that can be cured by overexpression of Ure2p fragments. Proc. Natl. Acad. Sci. U. S. A. 96, 1498 1503. 8. Moriyama, H., Edskes, H.K., and Wickner, R.B. (2000). URE3 prion propagation in Saccharomyces cerevisiae: requirement for chaperone Hsp104 and curing by overexpressed chaperone Ydj1p. Mol. Cell. Biol. 20, 8916 - 8922. 9. Kryndushkin, D., and Wickner, R.B. (2007). Nucleotide exchange factors for Hsp70s are required for URE3 prion propagation in Saccharomyces cerevisiae. Mol. Biol. Cell. 18, 2149 - 2154. 10. Kryndushkin, D. and Wickner, R.B. (2008) Curing of the URE3 prion by Btn2p, a Batten disease-related protein. EMBO J. 27, 2725-35.